FIG. 6.

Magneto-optical trap (MOT). (a) Three mutually orthogonal, counter-propagating pairs of laser beams are aimed at the trap center, defined by the zero of a quadrupole magnetic field created by a pair of coils with opposing currents ±I (anti-Helmoltz coils). The circularly polarized laser beams have either clockwise (CW) or counter clockwise (CCW) polarization, depending on the sign of the local magnetic field. (b) Energy diagram along one axis of the MOT, showing the Zeeman splitting of the magnetic sublevels of a J = 0 → J = 1 transition in the presence of a quadrupole magnetic field. With the quantization axis taken along the magnetic field direction, the $m_J^{\prime }=-1$ level becomes lower in energy and the $m_J^{\prime }=+1$ level becomes higher. CCW circularly polarized laser light incident from one side of the trap interacts only with $m_J^{\prime }=-1$ atoms on the incident side and only with $m_J^{\prime }=+1$ atoms on the other side, where the magnetic field direction is reversed. When the laser is tuned below resonance, the light force on the incident size of the trap is much stronger, creating a net restoring force toward the center of the trap.